1. Field of the Invention
This invention relates to a throttle device for a multipurpose engine, particularly to a throttle device for a multipurpose engine having an electric motor to open and close a throttle valve.
2. Description of the Related Art
The multipurpose engines used as prime movers in generators, agriculture machinery and various other applications have generally used a throttle device constituted as a mechanical governor. Specifically, engine speed is regulated by using a mechanical governor composed of weights and a spring to open and close a throttle valve housed in a throttle body.
While not relating to a multipurpose engine, Japanese Laid-Open Patent Application No. Hei 5(1993)-231894 (paragraphs 0012, 0013 and elsewhere), teaches a throttle device for an automobile engine, which improves the device in terms of structural simplicity, compactness and mountability by attaching an airflow sensor, throttle position sensor and control unit directly to the throttle body and enclosing them with a cover.
Attempts to improve fuel efficiency and reduce emissions have extended to the multipurpose engine in recent years. This led to the development of technologies for improving the accuracy of air intake regulation by using an electronically controlled throttle device (i.e., electronic governor) for opening and closing the throttle valve using an electric motor.
When a throttle valve is driven by an electric motor, the drive torque applied to the throttle valve needs to be increased and the step angle (minimum a rotation angle) needs to be decreased. This requires use of a speed reducer for gearing down the output of the motor before transmitting it to the throttle valve. A circuit board having an electronic control unit for controlling the operation of the motor, a harness for interconnecting the circuit board and the motor, and the like also become necessary.
As pointed out above, multipurpose engines are used to drive various kinds of equipment and when the engine is built into a machine it is subject to numerous layout restrictions, making it essential to avoid increase in the size of the incorporated components. Use of an electric motor for driving the throttle valve has therefore been disadvantageous because the large number of components required and the relatively large size thereof has tended to increase the overall size of the throttle device.